Delphi records assignment

Question

I've encountered some strange behaviour of delphi XE3 compiler (i compile for x86 architecture).

Imagine i have class with one field - custom record with several field of simple types:

  TPage = class
  type
    TParagraph = record
    public
      FOwner:  TPage;

      FFirst:  Integer;
      FSecond: Integer;

      procedure Select;
    end;
  public
    FSelected: TParagraph;
  end;

procedure TPage.TParagraph.Select;
begin
  FOwner.FSelected:=Self;
end;

The logic is my page can contain several paragraphs and in some moments i want one of these paragraphs become selected (to be able to do some actions with it in other parts of my programm):

procedure TMainForm.Button1Click(Sender: TObject);
var
  lcPage:      TPage;
  lcParagraph: TPage.TParagraph;
begin
  lcPage:=TPage.Create;
  try
    <...>

    lcParagraph.FOwner:=lcPage;
    lcParagraph.FFirst:=1;
    lcParagraph.FSecond:=2;

    lcParagraph.Select;

    <...>
  finally
    lcPage.Free;
  end;

Everything is ok when my record do not exceed some size. One reference and two integers is ok, in this case i get assembler instructions like this:

MainUnit.pas.350: FOwner.FSelected:=Self;
00C117B3 8B45FC           mov eax,[ebp-$04]
00C117B6 8B00             mov eax,[eax]
00C117B8 8B55FC           mov edx,[ebp-$04]
00C117BB 8B0A             mov ecx,[edx]
00C117BD 894804           mov [eax+$04],ecx
00C117C0 8B4A04           mov ecx,[edx+$04]
00C117C3 894808           mov [eax+$08],ecx
00C117C6 8B4A08           mov ecx,[edx+$08]
00C117C9 89480C           mov [eax+$0c],ecx

I can see three correct movs which copies memory from local record to class`.

But! If i add more fields to my record, the generated asm code changes and the record assignment no longer performs correctly.

    TParagraph = record
    public
      FOwner:  TPage;

      FFirst:  Integer;
      FSecond: Integer;
      FThird:  Integer;

      procedure Select;
    end;

MainUnit.pas.350: FOwner.FSelected:=Self;
00C117C9 8B45FC           mov eax,[ebp-$04]
00C117CC 8B55FC           mov edx,[ebp-$04]
00C117CF 8B12             mov edx,[edx]
00C117D1 8BF2             mov esi,edx
00C117D3 8D7A04           lea edi,[edx+$04]
00C117D6 A5               movsd 
00C117D7 A5               movsd 
00C117D8 A5               movsd 
00C117D9 A5               movsd 

And then i get garbage in class' record FSelected:

After the lea instuction the CPU state is such:

In this example 02D37280 is the address of my lcPage class, so 02D37284 should contain start of its field - FSelected record. But movsd instruction copies memory from ESI to EDI, from 02D37280 to 02D37284, which make absolutly no sense! If i change ESI register to value of EAX (19F308), which is start of my local lcParagraph variable, the copy is performed properply:

Is what i described is known bug? Or am i missing something fundamental about delphi? Is this a good way to assign records? I can easily workaround the problem, for example, by changing FOwner.FSelected:=Self; to CopyMemory(@FOwner.FSelected, @Self, SizeOf(Self)); in procedure TPage.TParagraph.Select;. But i want to figure out what is wrong.

Minimal reproducible example:

program RecordAssignmentIssue;

{$APPTYPE CONSOLE}

{$R *.res}

uses
  System.SysUtils;

type
  TPage = class
  type
    TParagraph = record
    public
      FOwner:  TPage;

      FFirst:  Integer;
      FSecond: Integer;
      FThird:  Integer;

      procedure Select;
    end;
  public
    FSelected: TParagraph;
  end;

procedure TPage.TParagraph.Select;
begin
  FOwner.FSelected:=Self;
end;

var
  lcPage:      TPage;
  lcParagraph: TPage.TParagraph;
begin
  try
    lcPage:=TPage.Create;
    try
      lcParagraph.FOwner:=lcPage;
      lcParagraph.FFirst:=1;
      lcParagraph.FSecond:=2;
      lcParagraph.FThird:=3;

      lcParagraph.Select;

      Assert(CompareMem(@lcPage.FSelected, @lcParagraph, SizeOf(lcParagraph)));
      // get rid of FThird and assertion will pass
    finally
      lcPage.Free;
    end;
  except
    on E: Exception do
      Writeln(E.ClassName, ': ', E.Message);
  end;
end.

Answer 1

This is a bug that is still present in Delphi 11 (thanks to LU RD for confirming that). You should submit a bug report to Quality Portal.

In the meantime, I think that you can work around it by making the assignment in TPage rather than TParagraph. Like this:

program RecordAssignmentIssue;

{$APPTYPE CONSOLE}

uses
  System.SysUtils;

type
  TPage = class
  type
    TParagraph = record
    public
      FOwner:  TPage;

      FFirst:  Integer;
      FSecond: Integer;
      FThird:  Integer;

      procedure Select;
    end;
  private
    procedure Select(const Paragraph: TParagraph);
  public
    FSelected: TParagraph;
  end;

procedure TPage.TParagraph.Select;
begin
  FOwner.Select(Self);
end;

{ TPage }

procedure TPage.Select(const Paragraph: TParagraph);
begin
  FSelected:=Paragraph;
end;

var
  lcPage:      TPage;
  lcParagraph: TPage.TParagraph;

begin
  try
    lcPage:=TPage.Create;
    try
      lcParagraph.FOwner:=lcPage;
      lcParagraph.FFirst:=1;
      lcParagraph.FSecond:=2;
      lcParagraph.FThird:=3;

      lcParagraph.Select;

      Assert(CompareMem(@lcPage.FSelected, @lcParagraph, SizeOf(lcParagraph)));
      // get rid of FThird and assertion will pass
    finally
      lcPage.Free;
    end;
  except
    on E: Exception do
      Writeln(E.ClassName, ': ', E.Message);
  end;
end.

Or another very simple workaround is just to introduce an extra local pointer variable to hold a pointer to Self:

procedure TPage.TParagraph.Select;
var
  P: ^TParagraph;
begin
  P := @Self;
  FOwner.FSelected := P^;
end;